Illumination assemblies are used in a variety of diverse applications. Traditional illumination assemblies have used lighting sources such as incandescent or fluorescent lights. More recently, other types of light emitting elements, and light emitting diodes (LEDs) in particular, have been used in illumination assemblies. LEDs have the advantages of small size, long life, and energy efficiency. These advantages of LEDs make them useful in many diverse applications.
For many lighting applications, it is desirable to have one or more LEDs supply the required luminous flux and/or illuminance. LEDs in an array are commonly connected to each other and to other electrical systems by mounting the LEDs onto a substrate. LEDs may be populated onto a substrate using techniques that are common to other areas of electronics manufacturing, e.g., locating components onto circuit board traces, followed by bonding the components to the substrate using one of a number of known technologies, including hand soldering, wave soldering, reflow soldering, and attachment using conductive adhesives.
In addition to light, LEDs generate heat during operation. The amount of heat and light generated by an LED is generally proportional to the current flow. Consequently, the more light an LED generates, the more heat the LED generates. Unfortunately, as LED current increases and temperature increases, less light is produced proportional to current, causing LED efficiency and lifetime to decrease.
One prior art attempt to reduce the total heat in a lighting system is shown schematically in FIG. 1. The lighting system 1 of FIG. 1 includes multiple LEDs 2 affixed to a substrate 3. Multiple solid fins 4 are vertically attached to substrate 3. Heat generated by each LED 2 is diffused to substrate 3 and further into solid fins 4. Air flow around solid fins 4 causes convective cooling of solid fins 4.
Another prior art attempt to reduce the total heat in a lighting system is shown schematically in FIG. 2. The lighting system 5 of FIG. 2 is the same as lighting system 1 of FIG. 1 except that multiple heat pipes 6 are embedded in or attached to substrate 3 such that substrate 3 effectively becomes a heat spreader. A heat pipe is a heat transfer device that can transport large quantities of heat with a very small difference in temperature between hotter and colder interfaces. Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant.
Planar heat pipe (or heat spreader) 6 as shown in FIG. 2 includes a hermetically sealed hollow vessel containing a working fluid (not shown) and a closed-loop capillary recirculation system (not shown). Inside the walls of heat pipe 6, at the hotter interface(s), the working fluid turns to vapor, which naturally flows and condenses on the colder interface(s). The liquid falls or is moved by capillary action back to the hot interface to evaporate again and repeat the cycle. One practical limit to the rate of heat transfer is the speed with which the gas can be condensed to a liquid at the cold end. When one end of the heat pipe is heated, the working fluid inside the pipe at that end evaporates and increases the vapor pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporization of the working fluid reduces the temperature at the hot end of the pipe. The vapor pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapor pressure over condensing working fluid at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapor condenses, releases its latent heat, and warms the cool end of the pipe. In this way, heat from LEDs 2 is dissipated throughout lighting system 5.
Another prior art attempt to reduce the total heat in a lighting system is shown schematically in FIG. 3. The lighting system 7 of FIG. 3 includes multiple LEDs 2 attached to the underside of substrate 3. Two heat pipes 6 are attached to substrate 3 and curve upward. Multiple solid fins 4 are attached to each heat pipe 6. Heat generated by LEDs 2 diffuses to substrate 3, then to heat pipes 6, and then to fins 4 which rely on convective cooling.